Toxoplasma gondii is associated with morbidity and mortality in a variety of marine mammals, including numerous fatal brain infections in the southern sea otter (Enhydra lutris nereis). The source(s) of T. gondii introduction and mechanisms of transmission in the marine environment are unknown. We hypothesized that filter-feeding marine bivalve shellfish serve as phoretic hosts by concentration of infective T. gondii oocysts and subsequent predation by southern sea otters. We have developed a real-time PCR assay and validated its use in detection of T. gondii 18s RNA in experimentally-exposed mussels (Mytilus galloprovincialis) under laboratory conditions. T. gondii-specific RNA was detected in mussels as long as 21 days post-exposure. Parasite RNA was most often detected in digestive gland homogenate (31/35 or 89 percent). Oocyst infectivity was confirmed using a mouse bioassay. Infections were detected in mice receiving any one of the mussel sample preparations (hemolymph, gill or digestive gland), but only digestive gland remained bioassay-positive over the longer post-exposure periods. For each time point, the total proportion of mice inoculated with each of the different tissues from T. gondii-exposed mussels was similar to the proportion of exposed mussels from the same treatment groups that were positive via PCR. This TaqMan system, which detects multiple genotypes of T. gondii, is being adapted for use in field sampling wild invertebrate prey species from coastal hot spots where T. gondii infections appear to be common in southern sea otters.
This research was funded by the National Sea Grant Program (NA06RG0142 R/CZ-169). The authors wish to thank Anne McBride and Holly Mendonca (UC Bodega Marine Laboratory) and Erin Dodd, Heather Harris, Christine Arnold, Jack Hawkes, and Debbie Brownstein (CDFG) for their assistance with the experiments and sample preparation.